1. Field of the Invention
The present invention relates to a positive photoresist composition which exhibits high sensitivity and definition, and improved focal depth range properties and underexposure margin. The term xe2x80x9cunderexposure marginxe2x80x9d used herein means the ratio Eop/Es, where the exposure to exactly reproduce a pattern in accordance with predetermined dimensions by a mask pattern at an illumination intensity at substrate of 500 mW/cm2 is defined as the exposure Eop (ms) and the minimum exposure to give an isolated pattern at an illumination intensity at substrate of 500 mW/cm2 is defined as the exposure Es (ms).
2. Description of the Related Art
In the manufacture of ultralarge-scale integrated circuit (Ultra LSI) where a high definition of not more than half a micron, in particular, of not more than 0.35 xcexcm is required, there are demands on photoresist compositions which can form a resist pattern satisfactory in sensitivity, definition, underexposure margin, focal depth range properties and having a good shape.
By way of illustration, Japanese Patent Laid-Open No. 6-167805 discloses a positive photoresist composition containing a quinonediazide ester having a tetra- to hepta-nuclear linear polyphenol skeleton as a photosensitizer. This conventional composition has high definition but remarkably low underexposure margin of 1.0 to 1.2 due to its extremely high xcex3 value, and can hardly form a pattern particularly at positions with low exposures (underexposure side).
In the exposure step, therefore, the formation of a resist pattern is blocked when the exposure is shortened for only several milliseconds relative to the optimum exposure (exposure Eop) or when thickness of a resist film becomes only several hundred nanometers thicker at locations corresponding to steps of a substrate.
In addition, a material containing the aforementioned photosensitizer hardly forms an isolated pattern when the focus of a light source in the exposure step shifts to the plus side (i.e., the light focus is in the substrate side from the resist surface) in the formation of an ultrafine resist pattern of not more than 0.35 xcexcm. FIG. 4 demonstrates changes in the formation of resist patterns with shifts of the focus, where (a) indicates the case using the aforementioned photosensitizer, and (b) indicates the case using another photosensitizer having no tetra- to hepta-nuclear linear skeleton (e.g., a photosensitizer described in Japanese Patent Laid-Open No. 7-152151 below). FIG. 4 demonstrates that the pattern formations in both cases are similar when the focus shifts to the minus side from the zero point where the focus is not shifted, but the use of the aforementioned photosensitizer fails to form any pattern when the focus shifts to the plus side, which is inherent to this photosensitizer.
As a result, the photosensitizer is disadvantageous in that it is narrow in focal depth range and cannot form a resist pattern of the predetermined dimensions when the distance between a light source and a substrate shifts in the exposure step or when a resist film is formed on a stepped substrate and hence the surface of resist film is uneven due to steps of the substrate.
Japanese Patent Laid-Open No. 7-152151 discloses a positive photoresist composition containing an alkali-soluble resin, a quinonediazide ester and a 4,4xe2x80x2-bis(dialkylamino)benzophenone, as a material exhibiting high definition and satisfactory focal depth range properties. This composition is, however, low in underexposure margin.
Accordingly, it is an object of the present invention to provide a positive photoresist composition which exhibits high sensitivity and high definition and improved focal depth range properties and underexposure margin.
After intensive investigations, the present inventors have solved the above problems.
To be more specific, the present invention provides a positive photoresist composition comprising (A) an alkali-soluble resin, (B) a quinonediazide ester of at least one compound selected from compounds represented by the following formula (I): 
wherein each of R1 through R8 is independently a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, and the following formula (II): 
wherein each of R9 through R17 is independently a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, and R18 is an alkyl group having 1 to 3 carbon atoms or a cycloalkyl group; and (C) 4,4xe2x80x2-bis(diethylamino)benzophenone.
The content of Ingredient (C) may preferably fall in the range from 0.1% to 1.0% by weight relative to the total solid contents in the composition other than Ingredient (C).
Ingredient (B) in the composition may be a quinonediazide diester of a compound of the following formula (Ia): 
The invention further provides a process for the formation of a resist pattern having a high underexposure margin Eop/Es, where the exposure to exactly reproduce a pattern in accordance with predetermined dimensions of a mask pattern at an illumination intensity at substrate of 500 mW/cm2 is defined as the exposure Eop (ms) and the minimum exposure to give an isolated pattern at an illumination intensity at substrate of 500 mW/cm2 is defined as the exposure (ms) Es, comprising the steps of coating a solution of the aforementioned positive photoresist composition onto a substrate, drying the coated substrate to form a photosensitive layer, exposing the photosensitive layer to light and developing it with a proper developer.